Polymerizable compositions made with polymerization initiator systems based on organoborane amine complexes

ABSTRACT

A polymerizable acrylic composition comprises: 
     (a) at least one acrylic monomer; 
     (b) an effective amount of an organoborane amine complex having the structure: ##STR1## wherein: R 1  is an alkyl group having 1 to 10 carbon atoms; 
     R 2  and R 3  are independently selected from phenyl-containing groups and alkyl groups having 1 to 10 carbon atoms; 
     R 4  is selected from the group consisting of CH 2  CH 2  OH and (CH 2 ) x  NH 2  wherein x is an integer greater than 2; 
     R 5  is hydrogen or an alkyl group having 1 to 10 carbon atoms; and 
     the nitrogen atom to boron atom ratio is about 1:1 to 1.5:1; and 
     (c) an effective amount of an acid for initiating polymerization of the acrylic monomer. 
     The polymerizable acrylic compositions are especially useful for bonding low surface energy substrates.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to organoborane amine complexes and,more particularly, to polymerizable compositions, especially acrylicadhesives, that incorporate polymerization initiator systems based onthe organoborane amine complexes. This invention further relates tomethods for bonding substrates, particularly low surface energysubstrates, using such compositions.

2. Description of the Related Art

An efficient, effective means for adhesively bonding low surface energysubstrates such as polyethylene, polypropylene andpolytetrafluoroethylene (e.g., TEFLON) has long been sought. Thedifficulties in adhesively bonding these materials are well known. See,for example, "Adhesion Problems at Polymer Surfaces" by D. M. Brewisthat appeared in Progress in Rubber and Plastic Technology, volume 1,page 1 (1985). The conventional approaches typically function by: (1)increasing the surface energy of the substrate (to more closely matchthe surface energies of the substrate and the adhesive thereby promotingbetter wetting of the substrate by the adhesive) and/or (2) eliminatingadditives and low molecular weight polymer fractions in the substratethat can migrate to the substrate surface and adversely affect adhesionby forming a weak boundary layer.

As a result, the conventional approaches often use complex and costlysubstrate surface preparation techniques such as flame treatment, coronadischarge, plasma treatment, oxidation by ozone or oxidizing acids, andsputter etching. Alternatively, the substrate surface may be primed bycoating it with a high surface energy material. However, to achieveadequate adhesion of the primer, it may be necessary to first use thesurface preparation techniques described above. All of these techniquesare well known, as reported in Treatise on Adhesion and Adhesives (J. D.Minford, editor, Marcel Dekker, 1991, New York, volume 7, pages 333 to435). The known approaches are frequently customized for use withspecific substrates. As a result, they may not be useful for bonding lowsurface energy substrates generally.

Moreover, the complexity and cost of the presently known approaches donot render them particularly suitable for use by the retail consumer(e.g., home repairs, do-it-yourselfers, etc.) or in low volumeoperations. One vexing problem is the repair of many inexpensiveeveryday household articles that are made of polyethylene, polypropyleneor polystyrene such as trash baskets, laundry baskets and toys.

Consequently, there has been a considerable and long felt need for asimple, easy to use adhesive that can readily bond a wide variety ofsubstrates, especially low surface energy materials, such aspolyethylene, polypropylene and polytetrafluoroethylene, withoutrequiring complicated surface preparation, priming and the like.

This invention is directed to polymerization initiator systems based onorganoborane amine complexes and adhesives and other compositions madetherewith. The adhesives are especially useful in bonding low surfaceenergy substrates such as polyethylene, polypropylene andpolytetrafluoroethylene.

In 1957 G. S. Kolesnikov et al. (Bull. Acad. Sci. USSR, Div. Chem. Sci.1957, p. 653) reported the use of tributylborane as a catalyst for thepolymerization of styrene and methyl methacrylate. The addition of 2mole % of tributylborane to methyl methacrylate resulted in rapidpolymerization; a transparent solid block was formed in 60 to 90minutes. At about the same time, J. Furakawa et al. (Journal of PolymerScience, volume 26, issue 113, p. 234, 1957) reported thattriethylborane had been found to initiate the polymerization of somevinyl compounds such as vinyl acetate, vinyl chloride, vinylidenechloride, methacrylic ester, acrylic ester, and acrylonitrile. J.Furakawa et al. (Journal of Polymer Science, volume 28, issue 116, 1958)later reported that triethyl borane-catalyzed vinyl polymerization couldbe markedly accelerated with oxygen or oxygen compounds such as hydrogenperoxide and metal oxides. While the presence of oxygen is apparentlyneeded for the polymerization to occur, the organoborane compounds ofthe type described in these references are known to be quite pyrophoricin air. Hence, the presence of oxygen is simultaneously required andundesirable.

U.S. Pat. No. 3,275,611 "Process for Polymerizing Unsaturated Monomerswith a Catalyst Comprising an Organoboron Compound, a Peroxygen Compoundand an Amine" issued Sep. 27, 1966 to E. H. Mottus et al. discloses aprocess for polymerizing olefinic compounds, especiallyalpha-olefinically unsaturated compounds. Particularly preferred aremethacrylate monomers having no more than 20 carbon atoms in the estergroup. The organoboron compound and the amine may be added to thereaction mixture separately or they may be added as a preformed complex.The latter approach reportedly has the advantage of making the boroncompound more easily handled, especially for certain boron compoundsthat tend to be pyrophoric in air but which are not pyrophoric whencomplexed. Especially useful boron catalysts are said to have thefollowing general formulas: R₃ B, RB(OR)₂, R₂ B(OR), R₂ BOBR₂, R₂ BX,and R₂ BH, where R is a hydrocarbon radical, preferably an alkyl radicalhaving from 1 to 10 or more carbon atoms (more preferably, up to 6carbon atoms), and X is a halogen.

Useful amine complexing agents are said to have a basicity that ispreferably in the range of about 10⁻⁶ or 10⁻⁷ to 5×10⁻¹⁰ or 10⁻¹⁰.Various amine complexing agents are mentioned although pyridine,aniline, toluidine, dimethylbenzylamine, and nicotine are used in theexamples. The amine and boron compounds are used in about a 1:1 molarratio, assuming one nitrogen function per boron function. Reportedly,any peroxide or hydroperoxide compound may be used as a catalystcomponent.

While Mottus et al. refer to polymerizing methacrylate monomers, thereis no indication that the resulting polymers are useful as adhesives.Various acids are mentioned as monomers that may be polymerized butthere is no indication that an acid is a component of the polymerizationsystem.

British Patent Specification No. 1,113,722 "Aerobically PolymerisableCompositions," published May 15, 1968 discloses the polymerization ofacrylate monomers through the use of a free-radical catalyst (e.g.,peroxides) and triarylborane complexes having the general formula(R₃)B-Am wherein R is an aryl radical having from 6 to 12 carbon atomsand Am is, among other things, an amine such as hexamethylenediamine orethanolamine. The polymerization is activated by heating or the additionof an acid. The resulting compositions are reportedly useful asadhesives.

Chemical Abstracts No. 88532r (volume 73, 1970) "Dental Self-curingResin" and the full text paper to which it refers report thattributylborane can be made stable in air by complexing it with ammoniaor certain amines (e.g., aniline, n-butylamine, piperidine,ethylenediamine) at a mole ratio of one and that the tributylborane canbe reactivated with an amine acceptor such as an isocyanate, an acidchloride, a sulfonyl chloride, or anhydrous acetic acid. As a result,the complex can be used to polymerize blends of methyl methacrylate andpoly(methylmethacrylate) to provide a dental adhesive.Tributylborane-ethylenediamine complexes and triethylborane-ammoniacomplexes, each with p-toluenesulfonyl chloride as the amine acceptor,are specifically mentioned.

Chemical Abstracts No. 134385q (volume 80, 1974) "Bonding Polyolefin orVinyl Polymers" reports that a mixture of 10 parts methyl methacrylate,0.2 parts tributylborane, and 10 parts poly(methylmethacrylate) was usedto bond polyethylene, polypropylene and. poly(vinyl acetate) rods.

U.S. Pat. No. 5,106,928 "Acrylic Adhesive Composition and OrganoboronInitiator System," issued Apr. 21, 1992 to M. M. Skoultchi et al.,discloses a two-part initiator system that is reportedly useful inacrylic adhesive compositions, especially elastomeric acrylic adhesives.The first part of the two-part initiating system is a stabilizingorganoborane amine complex; the second part is an organic acidactivator. The organoborane compound of the complex has the generalformula: ##STR2## where R, R₁ and R₂ are either alkyl groups having 1 to10 carbon atoms or phenyl groups, although alkyl groups of 1 to 4 carbonatoms are preferred. The amine portion of the complex may be ammonia, aprimary amine, a secondary amine, or a polyamine containing a primaryamine or a secondary amine. Useful amines include n-octylamine,1,6-diaminohexane, diethylamine, dibutylamine, diethylenetriamine,dipropylenediamine, 1,3-propylenediamine, and 1,2-propylenediamine.

The organic acid activator is a compound that will destabilize orliberate the free organoborane compound by removing the amine group,thereby allowing it to initiate the polymerization process. Preferably,the organic acid has the formula R--COOH where R is hydrogen, an alkylor alkenyl group having 1 to 8 (preferably 1 to 4) carbon atoms, or anaryl group with 6 to 10 (preferably 6 to 8) carbon atoms.

Twelve organoborane amine initiator complexes are described inconjunction with Example 1. In those complexes based on diamines ortriamines, the nitrogen atom to boron atom ratio ranges from 2:1 to 4:1.In those complexes based on diethylamine and n-octylamine, the nitrogenatom to boron atom ratio is 1.5:1.

The adhesive compositions are reportedly particularly useful instructural and semi-structural applications such as speaker magnets,metal-metal bonding, (automotive) glass-metal bonding, glass-glassbonding, circuit board component bonding, selected plastic to metal,glass, wood, etc. and electric motor magnets. Those plastics that may bebonded are not further described.

SUMMARY OF THE INVENTION

The invention relates to polymerizable acrylic compositions, especiallyacrylic adhesives, that incorporate polymerization initiator systemsbased on organoborane amine complexs. The adhesives are particularlyuseful in bonding low surface energy substrates (e.g., polyethylene,polypropylene, polytetrafluoroethylene, etc.) that, heretofore, havebeen bonded using complex and costly surface preparation techniques.

The polymerizable acrylic compositions comprise and, more preferably,consist essentially of at least one acrylic monomer (preferably alkylacrylates such as butylacrylate and/or alkyl methacrylates such asmethylmethacrylate), an effective amount of an organoborane aminecomplex, and an effective amount of an organic or inorganic acid (e.g.,acrylic acid, methacrylic acid or SnCl₄) for initiating polymerizationof the acrylic monomer.

Useful organoborane amine complexes have the following general formula:##STR3## wherein: R¹ is an alkyl group having 1 to 10 (preferably 2 to5) carbon atoms;

R² and R³ are independently selected from phenyl-containing groups andalkyl groups having 1 to 10 carbon atoms, alkyl groups having 2 to 5carbon atoms being preferred;

R⁴ is selected from the group consisting of CH₂ CH₂ OH and (CH₂)_(x) NH₂wherein x is an integer greater than 2, preferably from 2 to 6, and mostpreferably 6;

R⁵ is hydrogen (preferred) or an alkyl group having 1 to 10 carbonatoms; and

the nitrogen atom to boron atom ratio is about 1:1 to 2:1, morepreferably about 1:1 to 1.5:1, and most preferably about 1:1.

The organoborane amine complex is typically provided in an amount ofabout 0.15 to 3 mole % based on the number of moles of acrylic groups,moieties or functionality (more preferably about 0.2 to 2.5 mole %; mostpreferably about 1 to 1.5 mole %). An effective amount of the acid isabout 30 to 540 mole % (most preferably about 230 mole %), based on thenumber of equivalents of amine groups, moieties or functionality.

Among the useful additives that may be optionally included within thesecompositions are thickening agents (such as polymethylmethacrylate) anda small amount (about 0.1 to 7 mole % based on the number of moles ofacrylic functionality) of a substantially uncomplexed organoborane, thelatter being especially useful if the organoborane amine complex isbased on monoethanolamine.

In another aspect the invention relates to a method for bonding lowsurface energy polymeric substrates using the polymerizable acryliccompositions described above. The substrate surface may first be primedwith a composition comprising the organoborane amine complex in an inertorganic solvent (e.g., to about 5 to 15 wt. %), in which case theinclusion of the organoborane amine complex in the polymerizablecomposition is optional.

In yet another aspect of the invention, certain compositions are usefulas primers for increasing the adhesion of a subsequently appliedadhesive to fluoroplastic substrates. Among such useful primers arethose based on acrylic monomers, organoboranes and an oxygen source(e.g. peroxides or atmospheric oxygen) as well as those based on acrylicmonomers, organoborane amine complexes, and acids.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In a broad aspect, this invention relates to polymerizable acryliccompositions, especially acrylic adhesives, that are produced usingpolymerization initiator systems based on organoborane amine complexes.The adhesives are particularly useful in bonding low surface energysubstrates (e.g., polyethylene, polypropylene, polytetrafluoroethylene,etc.) that, heretofore, have been bonded using complex and costlysurface preparation techniques.

The polymerization initiator systems useful in the invention compriseand, more preferably, consist essentially of an effective amount of anorganoborane amine complex and an effective amount of an acid forliberating the organoborane to initiate polymerization. Organoboraneamine complexes useful in the invention have the following generalstructure: ##STR4## where R¹ is an alkyl group having 1 to 10 carbonatoms and R² and R³ are independently selected from alkyl groups having1 to 10 carbon atoms and phenyl-containing groups More preferably, R¹,R² and R³ are alkyl groups having 1 to 5 carbon atoms such as methyl,ethyl, propyl, iso-propyl, n-butyl, iso-butyl, and pentyl. In general,shorter carbon chain lengths are preferred for the R¹, R² and R³ groupsas this promotes enhanced stability of the complex in air. Smaller, lessbulky substituents are also preferred as larger, more bulky groups maynegatively affect adhesion. By "independently selected" it is meant thatR² and R³ may be the same or that they may be different. R¹ may be thesame as R² or R³, or it may be different. Preferably R¹, R² and R³ arethe same The tri-propyl, tri-iso-propyl, and tri-n-butyl alkylboraneshave been found to be especially useful.

The amine component of the complex may be either monoethanolamine, aprimary alkyl diamine, or a secondary alkyl diamine Consequently, R⁴ maybe selected from the group consisting of CH₂ CH₂ OH and (CH₂)_(x) NH₂,wherein x is an integer greater than 2. R⁵ is either hydrogen or analkyl group having 1 to 10 carbon atoms In more preferred complexes, R⁵is hydrogen (so as to reduce the steric hinderance within theorganoborane amine complex which could inhibit the formation of thecomplex itself) and R⁴ is either CH₂ CH₂ OH or (CH₂)_(x) NH₂ where x isan integer from 2 to 6. Most preferred, however, are complexes where theR⁴ is CH₂ CH₂ OH (monoethanolamine) or (CH₂)₆ NH₂(1,6-hexamethylenediamine).

Importantly, and as shown more fully hereinbelow, the nitrogen atom toboron atom ratio in the complex is about 1:1 to 2:1, more preferablyabout 1:1 to less than 2:1, even more preferably about 1:1 to 1.5:1, andmost preferably about 1:1. At nitrogen atom to boron atom ratios inexcess of 2:1 the practical utility of the complex in a polymerizationinitiator system is diminished as the amount of complex that must beemployed to achieve a useful molecular weight during polymerizationbecomes too large. On the other hand, a nitrogen atom to boron atomratio of less than 1:1 leaves free organoborane, a material that tendsto be pyrophoric.

An effective amount of the organoborane amine complex is an amount thatis large enough to permit polymerization to readily occur to obtain anacrylic polymer of high enough molecular weight for the desired end use.If the amount of organoborane amine complex is too high, then thepolymerization may proceed too rapidly to allow for effective mixing andapplication of the composition. The useful rate of polymerization willdepend in part on the method of applying the composition to thesubstrate. Thus, the rate of polymerization for a high speed automatedindustrial applicator can be faster than if the composition is appliedwith a hand applicator or if the composition is mixed manually.

Within these parameters, an effective amount of the organoborane aminecomplex is about 0.15 to 3 mole based on the number of moles of acrylicfunctionality, more preferably about 0.2 to 2.5 mole %, and mostpreferably about 1 to 1.5 mole %. If the amine is provided bymonoethanolamine, it has been found that an effective amount of thecomplex is greater than 2 mole % but less than about 5 mole %. By"acrylic functionality" is meant acrylic and substituted acrylicmoieties or chemical groups; that is, groups which have the generalstructure ##STR5## wherein R and R' are organic radicals that may be thesame or that may be different.

As explained below, however, it is sometimes advantageous to furtherinclude a small amount of additional, substantially uncomplexedorganoborane. In these instances, an effective amount of the complex isabout 0.3 to 5 mole %, based on the number of moles of acrylicfunctionality, more preferably about 0.5 to 4 mole %, and mostpreferably about 1 to 3 mole %.

Advantageously, the organoborane amine complexes useful in the inventionare air stable. By "air stable" it is meant that when the complexes arestored in a capped vessel at room temperature (about 20° to 22° C.) andunder otherwise ambient conditions (i.e., not under a vacuum and not inan inert atmosphere), the complexes remain useful as polymerizationinitiators for at least about two weeks, although the complexes may bereadily stored under these conditions for many months and up to a yearor more. By "air stable" it is also meant that the complexes are notpyrophoric, as explained more fully hereinbelow. The air stability ofthe complex is enhanced if it is provided as a crystalline material. Inits most stable form, the complex exists as clear white, solid,needlelike crystals. However, the complex is still useful even if it isprovided as an amorphous solid or a viscous, syrup-like liquid. Overtime, the most preferred clear white, solid, needlelike crystals mayassume these forms.

The organoborane amine complex may be readily prepared using knowntechniques. Typically, the amine, if provided as a solid, is ground to afine powder (preferably in an inert atmosphere) and combined with theorganoborane (also in an inert atmosphere) with slow stirring. Anexotherm is often observed and cooling of the mixture is, therefore,recommended. Due to the high vapor pressure of some of the materialsthat may be used, it is desirable to keep the reaction temperature belowabout 70° to 80° C., but the temperature should not be kept so low thatthe reaction product prematurely crystallizes. Once the materials havebeen well mixed, the complex is permitted to cool so that crystalsthereof may form. No special storage conditions are required although itis preferred that the complex be kept in a capped vessel in a cool, darklocation. Advantageously, the complexes used in the invention areprepared in the absence of organic solvents that would later have to beremoved.

Turning now to the acid, this component liberates the organoborane byremoving the amine group thereby permitting the organoborane to initiatepolymerization. Any acid that can liberate the organoborane by removingthe amine group may be employed. Useful acids include Lewis acids (e.g.,SnCl₄, TiCl₄ and the like) and Bronsted acids such as those having thegeneral formula R₆ -COOH, where R⁶ is hydrogen, an alkyl group, or analkenyl group of 1 to 8 and preferably 1 to 4 carbon atoms, or an arylgroup of 6 to 10, preferably 6 to 8 carbon atoms. The alkyl and alkenylgroups may comprise a straight chain or they may be branched. Suchgroups may be saturated or unsaturated. The aryl groups may containsubstituents such as alkyl, alkoxy or halogen moieties. Illustrativeacids of this type include acrylic acid, methacrylic acid, acetic acid,benzoic acid, and p-methoxybenzoic acid. Other useful Bronsted acidsinclude HCl, H₂ SO₄, H₃ PO₄ and the like. SnCl₄, acrylic acid andmethacrylic acid are preferred.

The acid should be used in an amount effective to promotepolymerization. If too little acid is employed, the rate ofpolymerization may be too slow and the monomers that are beingpolymerized may not adequately increase in molecular weight. However, areduced amount of acid may be helpful in slowing the rate ofpolymerization. On the other hand, if too much acid is used, then thepolymerization tends to proceed too quickly and, in the case ofadhesives, the resulting materials may demonstrate inadequate adhesionto low energy surfaces. On the other hand, an excess of acid may promoteadhesion to higher energy surfaces. Within these parameters, the acidshould, preferably, be provided in an amount of about 30 to 540 mole %based on the number of equivalents of amine functionality in thecomplex, more preferably about 100 to 350 mole %, and most preferablyabout 150 to 250 mole %. In the case of methacrylic acid and anorganoborane amine complex based on tripropylborane and1,6-hexamethylenediamine, about 0.5 to 7 wt. %, more preferably about 3wt. % based on the total weight of the composition, has been found to beuseful.

The organoborane amine complex initiator systems are especially usefulin polymerizing acrylic monomers, particularly for making polymerizableacrylic adhesives. By "acrylic monomer" is meant polymerizable monomershaving one or more acrylic or substituted acrylic moieties, chemicalgroups or functionality; that is, groups having the general structure##STR6## wherein R and R' are organic radicals that may be the same orthat may be different. Blends of acrylic monomers may also be used. Thepolymerizable acrylic monomer may be monofunctional, polyfunctional or acombination thereof.

The most useful monomers are monofunctional acrylate and methacrylateesters and the substituted derivatives thereof such as hydroxy, amide,cyano, chloro, and silane derivatives. Such monomers include, methylacrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate,isobornyl methacrylate, hydroxyethyl methacrylate, hydroxypropylacrylate, hydroxypropyl methacrylate, butyl acrylate, n-octyl acrylate,2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, decylmethacrylate,dodecyl methacrylate, cyclohexyl methacrylate, tert-butyl methacrylate,acrylamide, N-methyl acrylamide, diacetone acrylamide, N-tert-butylacrylamide, N-tert-octyl acrylamide, N-butoxyacrylamide,gamma-methacryloxypropyl trimethoxysilane, 2-cyanoethyl acrylate,3-cyanopropyl acrylate, tetrahydrofurfuryl methacrylate,tetrahydrofurfuryl chloroacrylate, glycidyl acrylate, glycidylmethacrylate, and the like. Dimethylaminoethyl acrylate anddimethylamino methacrylate may be used.

Particularly preferred are blends of alkyl acrylates (e.g., butylacrylate) and alkyl methacrylates (e.g., methyl methacrylate). Suchpolymerizable compositions according to the invention may broadlycomprise, based on the total weight of the composition, about 10 to 60wt. % (more preferably about 30 to 40 wt. %) of the alkyl methacrylate,and about 10 to 50 wt. % (more preferably about 25 to 35 wt. %) of thealkyl acrylate.

Another useful class of polymerizable monomers corresponds to thegeneral formula: ##STR7## R⁷ may be selected from the group consistingof hydrogen methyl, ethyl, --CH₂ OH, and ##STR8## R⁸ may be selectedfrom the group consisting of chlorine, methyl and ethyl. R⁹ may beselected from the group consisting of hydrogen, hydroxy, and ##STR9##The value of a is an integer greater than or equal to 1, morepreferably, from 1 to about 8, and most preferably from 1 to 4. Theintegral value of b is greater than or equal to 1, more preferably, from1 to about 20. The value of c is 0 or 1.

Acrylic monomers useful with the polymerization initiator systemsinclude ethylene glycol dimethacrylate, ethylene glycol diacrylate,polyethylene glycol diacrylate, tetraethylene glycol dimethacrylate,diglycerol diacrylate, diethylene glycol dimethacrylate, pentaerythritoltriacrylate, trimethylolpropane trimethacrylate, and other polyetherdiacrylates and dimethacrylates.

Other polymerizable monomers useful in the invention have the generalformula: ##STR10## R¹⁰ may be hydrogen, chlorine, methyl or ethyl; R¹¹may be an alkylene group with 2 to 6 carbon atoms; and R¹² is (CH₂)_(e)in which e is an integer of 0 to 8, or one of the following: ##STR11##the phenyl group being substitutable at any one of the ortho, meta orpara positions. The value of d is an integer of 1 to 4.

Typical monomers of this class include dimethacrylate of bis(ethyleneglycol) adipate, dimethacrylate of bis(ethylene glycol) maleate,dimethacrylate of bis(ethylene glycol) phthalate, dimethacrylate ofbis(tetraethylene glycol) phthalate, dimethacrylate of bis(tetraethyleneglycol) sebacate, dimethacrylates of bis(tetraethylene glycol) maleate,and the diacrylates and chloroacrylates corresponding to thedimethacrylates, and the like.

Also useful are monomers that are isocyanate-hydroxyacrylate orisocyanate-aminoacrylate reaction products. These may be characterizedas acrylate terminated polyurethanes and polyureides or polyureas. Suchmonomers have the following general formula: ##STR12## where X isselected from the group consisting of --O-- and ##STR13## R¹³ isselected from the group consisting of hydrogen and lower alkyl groups(i.e., 1 to 7 carbon atoms). T is the organic residue of an activehydrogen-containing acrylic ester the active hydrogen having beenremoved and the ester being hydroxy or amino substituted on the alkylportion thereof (including the methyl, ethyl and chlorine homologs). Theintegral value of f is from 1 to 6. L is a mono- or polyvalent organicradical selected from the group consisting of alkyl, alkylene, alkenyl,cycloalkyl, cycloalkylene, aryl, aralkyl, alkaryl, poly(oxyalkylene),poly(carboalkoxyalkylene), and heterocyclic radicals, both substitutedand unsubstituted.

Typical monomers of this class include the reaction product of mono- orpolyisocyanates, for example, toluene diisocyanate, with an acrylateester containing a hydroxy or an amino group in the non-acrylate portionthereof, for example, hydroxyethyl methacrylate.

Still another class of monomers useful in the present application arethe mono- and polyacrylate and methacrylate esters of bisphenol typecompounds. These monomers may be described by the following formula:##STR14## where R¹⁴ is methyl, ethyl, carboxyalkyl or hydrogen; R¹⁵ ishydrogen, methyl or ethyl; R¹⁶ is hydrogen, methyl or hydroxyl; R¹⁷ ishydrogen, chlorine, methyl or ethyl; and g is an integer having a valueof 0 to 8.

Representative monomers of the above-described class includedimethacrylate and diacrylate esters of 4,4'-bis-hydroxyethoxy-bisphenolA, dimethacrylate and diacrylate esters of bisphenol A, etc.

The compositions may further comprise a variety of optional additives.One particularly useful additive is a thickener such as a low (i.e.,less than or equal to about 100,000) molecular weight polymethylmethacrylate which may be incorporated in an amount of about 20 to 40wt. % (weight percent), based on the weight of the composition.Thickeners may be employed to increase the viscosity of the compositionto a more easily applied viscous syrup-like consistency.

Another useful adjuvant is a crosslinking agent that can be used toenhance the solvent resistance of the adhesive bond. Typically employedin an amount of about 0.2 to 1 weight percent based on the weight of thecomposition, useful crosslinkers include ethylene glycol dimethacrylate,ethylene glycol diacrylate, triethyleneglycol dimethacrylate, diethyleneglycol bismethacryloxy carbonate, polyethylene glycol diacrylate,tetraethylene glycol dimethacrylate, diglycerol diacrylate, diethleneglycol dimethacrylate, pentaerythritol triacrylate, trimethylopropanetrimethacrylate, and other polyether diacrylates and dimethacrylates.

Peroxides may be optionally included to adjust the speed at which thecompositions polymerize or to complete the polymerization.

Small amounts of inhibitors such as hydroquinone may be used to preventor reduce degradation of the acrylic monomers during storage. Inhibitorsmay be added in an amount that does not materially reduce the rate ofpolymerization or the adhesive properties of an adhesive made therewith,typically about 0.1 to 5% based on the weight of the polymerizablemonomers.

Various plasticizers and elastomeric fillers (i.e., rubbery polymersbased on polyisoprene, polybutadiene, polyolefins, polyurethanes andpolyesters) may be added to improve flexibility or toughness. Otherpossible additives include non-reactive colorants, fillers (e.g., carbonblack), etc. The optional additives are employed in an amount that doesnot significantly adversely affect the polymerization process or thedesired properties of compositions made therewith.

As will be shown below, the polymerizable acrylic compositions of theinvention are especially useful for adhesively bonding low surfaceenergy substrates that historically have been very difficult to bondwithout using complicated surface preparation techniques, priming, etc.By low surface energy substrates is meant materials that have a surfaceenergy of less than 45 mJ/m², more typically less than 40 mJ/m² or lessthan 35 mJ/m². Included among such materials are polyethylene,polypropylene, acrylonitrile-butadiene-styrene, polyamide, andfluorinated polymers such as polytetrafluoroethylene (TEFLON) which hasa surface energy of less than 20 mJ/m². Other polymers of somewhathigher surface energy that may be usefully bonded with the compositionsof the invention include polycarbonate and polymethylmethacrylate.However, the invention is not so limited; the compositions may be usedto bond any thermoplastic as well as wood, ceramics, concrete and primedmetals.

The polymerizable compositions of the invention are easily used astwo-part adhesives. The components of the polymerizable composition areblended as would normally be done when working with such materials. Theacid component of the polymerization initiator system is usuallyincluded in this blend so as to separate it from the organoborane aminecomplex, thus providing one part of the two-part composition. Theorganoborane amine complex of the polymerization initiator systemprovides the second part of the composition and is added to the firstpart shortly before it is desired to use the composition. The complexmay be added to the first part directly or it may be predissolved in anappropriate carrier such as a small amount of methyl methacrylate. Oncethe two parts have been combined, the composition should be usedquickly, as the useful pot life may be on the order of about aquarter-hour or so depending upon the monomer mix, the amount ofcomplex, and the temperature at which the bonding is to be performed.

The polymerizable composition is applied to one or both substrates andthen the substrates are joined together with pressure to force excesscomposition out of the bond line. This also has the advantage ofdisplacing composition that has been exposed to air and that may havebegun to oxidize. In general, the bonds should be made shortly after thecomposition has been applied, preferably within about 10 minutes. Thetypical bond line thickness is about 0.1 to 0.3 mm. The bonding processcan easily be carried out at room temperature and to improve the degreeof polymerization it is desirable to keep the temperature below about40° C., preferably below 30° C., and most preferably below about 25° C.

The bonds will cure to a reasonable green strength to permit handling ofthe bonded components within about 2 to 3 hours. Full strength will bereached in about 24 hours under ambient conditions; post-curing withheat may be used if desired.

When bonding fluoroplastics, it is advantageous to cool the first partof the two-part composition to about 0° to 5° C. before adding theorganoborane amine complex. The bond should be made as soon after thecomposition has been applied as practical; performing the bondingoperation at less than room temperature is also helpful.

The polymerization initiator systems are also very useful in formingprimers. A primer solution may be prepared by dissolving theorganoborane amine complex in an inert organic solvent such as pentane,hexane, petroleum ether, white spirits, benzene, toluene, ethylacetate,butylacetate, and the like. While any of the above describedorganoborane amine complexes may be used in making primers, those whichdemonstrate additional stability in organic solvents, such as thosecomplexes based on tripropylborane, are preferred.

An effective amount of the complex is a concentration of about 5 to 15wt. % in the solvent, preferably about 10 wt. %. A 10 wt. % primersolution applied at about 80 to 100 g/m² is adequate. If theconcentration is too low, then there is insufficient primer toeffectively polymerize the subsequently applied acrylic composition. Ifthe concentration is too high, then the polymerization may proceed tooquickly. In either event, the resulting adhesive bond may demonstratereduced shear adhesion.

The primer should be applied to the surfaces of both substrates that areto be bonded, although the subsequently applied acrylic composition needonly be provided on one surface. Once the solvent has been evaporated,the composition is desirably applied as soon after the primer has beendeposited as practically possible so as to avoid oxidative degradationof the primer. However, the use of the primer offers the distinctadvantage of permitting the application of the acrylic composition to bedelayed for several hours, as much as about 7 hours or even more.Otherwise, the bonding process is as described above with the use of thetwo-part compositions.

In addition to their outstanding utility as adhesives, the polymerizableacrylic compositions of the invention may be used as sealants, coatings,and injection molding resins. They may also be used as matrix resins inconjunction with glass and metal fiber mats such as in resin transfermolding operations. They may further be used as encapsulants and pottingcompounds such as in the manufacture of electrical components, printedcircuit boards and the like.

The invention will be more fully appreciated with reference to thefollowing nonlimiting examples in which all weights are given as weightpercents (wt. %) based on the total weight of the composition which is100 wt. %. Data reported in the following examples have been rounded offto one significant digit following the decimal. Accordingly, not allcompositions may sum to exactly 100.0%.

EXAMPLES 1 TO 13

Examples 1 to 13 illustrate the pyrophoricity of various organoboranesand organoborane amine complexes. The organoborane amine complexes wereprepared by combining the organoborane and the amine in an inert argonatmosphere with cooling to form the complex. The pyrophoricity of thevarious organoboranes and organoborane amine complexes was assessed in a"Charring Time" test and in an "Ignition Time" test.

The charring time was determined by applying one drop of theorganoborane or the organoborane amine complex to a 30 mm×30 mm piece ofcotton fabric and measuring the time that elapsed until the fabric beganto char or ignite (whichever first occurred). The ignition time wasdetermined by dipping another 30 mm×30 mm piece of cotton fabric intothe organoborane or the organoborane amine complex in an inertatmosphere, exposing the fabric to air, and measuring the time thatelapsed until the fabric ignited. The tests were terminated after about24 hours if no charring or ignition had occurred. The test results arereported below in Table 1.

Terms used in these examples are defined according to the followingschedule:

    ______________________________________                                               Term         Definition                                                ______________________________________                                               Bu           Butyl                                                            i-Bu         iso-Butyl                                                        Et           Ethyl                                                            Pr           Propyl                                                    ______________________________________                                    

                  TABLE 1                                                         ______________________________________                                               Organoborane or              Ignition                                  Example                                                                              Organoborane Amine                                                                            Charring Time                                                                              Time                                      No.    Complex         (seconds)    (seconds)                                 ______________________________________                                        1      Pr.sub.3 B       1*          1                                         2      i-Bu.sub.3 B    1            1                                         3      Bu.sub.3 B      1            1                                         4      Pr.sub.3 B.NH.sub.3                                                                           4-6          10                                        5      Pr.sub.3 B.HNEt.sub.2                                                                          3*          1                                         6      Pr.sub.3 B.H.sub.2 NBu-i                                                                      55           104                                       7      i-Bu.sub.3 B.NH.sub.3                                                                         4            4                                         8      i-Bu.sub.3 B.HNEt.sub.2                                                                       2            1                                         9      i-Bu.sub.3 B.H.sub.2 NBu-i                                                                    12           20                                        10     2Pr.sub.3 B.H.sub.2 N(CH.sub.2).sub.6 NH.sub.2                                                Did not char Did not                                                          or ignite    ignite                                    11     2i-Bu.sub.3 B.H.sub.2 N(CH.sub.2).sub.6 NH.sub.2                                              Did not char Did not                                                          or ignite    ignite                                    12     Pr.sub.3 B.H.sub.2 N(CH.sub.2).sub.6 NH.sub.2                                                 Did not char Did not                                                          or ignite    ignite                                    13     i-Bu.sub.3 B.H.sub.2 N(CH.sub.2).sub.6 NH.sub.2                                               Did not char Did not                                                          or ignite    ignite                                    ______________________________________                                         *Ignited.                                                                

Table 1 shows that uncomplexed organoboranes (examples 1 to 3) areinherently extremely pyrophoric and that complexing these materials withammonia, diethylamine, or isobutyl amine (examples 4 to 9) does notsufficiently reduce their inherent pyrophoricity to render the resultingcomplex readily usable. However, in examples 10 to 13 where theorganoboranes were complexed with 1,6-hexamethylenediamine, the cottonfabric neither charred nor ignited. The organoborane amine complexes ofexamples 10 to 13 are not pyrophoric and remain stable for at leastabout 2 weeks when stored in a sealed vessel at room temperature underotherwise ambient conditions. Thus, the organoborane amine complexes ofexamples 10 to 13 are "air stable" and are useful in providingpolymerization initiator systems and compositions made therewithaccording to the invention.

EXAMPLES 14 TO 53

Examples 14 to 53 show the excellent adhesion to low surface energysubstrates such as polytetrafluoroethylene (PTFE) and polyethylene (PE)that is possible when polymerizable acrylic adhesive compositions thatincorporate polymerization initiator systems are used.

Except as noted below, in each example a methacrylate monomer, anacrylate monomer, and a thickening agent ("Thickener") were stirredtogether until complete dissolution occurred using heat as necessary topromote dissolution. An acid, an organoborane amine complex, and anadditional amount of substantially uncomplexed organoborane were thenadded and mixed. Within about 10 minutes of the compositions having beenprepared they were applied to polytetrafluoroethylene and polyethylenesubstrates and tested for overlap shear strength at room temperaturefollowing the procedures of State Standard of the (Former) Soviet Union(GOST) 14759-69 ("Adhesive joints of metals. Method for determining theshear strength.").

More specifically, and unless noted otherwise, the composition wasapplied to substrate coupons measuring 60 mm×20 mm×2 mm thick. Thecoupons were mated to provide a 200 mm² overlap area and an adhesivebond that was about 0.1 to 0.3 mm thick. The bonded samples were usuallycured for about 24 hours before being mounted in a tensile testingapparatus and evaluated at a crosshead speed of 20 mm/min. The reporteddata are an average of five samples. The test results in megaPascals(MPa) are reported below in Table 2.

For polyethylene bonding, overlap shear strength values below about 5MPa are not generally considered desirable, values in the range of about5 to 8 MPa are marginally acceptable, and values greater than about 8are considered excellent. For polytetrafluoroethylene bonding, overlapshear strength values below about 3 MPa are generally consideredundesirable, values in the range of about 3 to 5 MPa are regarded asmarginally acceptable, and values in excess of 5 MPa are consideredexcellent.

Also reported in Table 2 is the failure mode for the various bondedcomposites. "A" refers to adhesional failure (i.e., failure at theinterface between the substrate and the adhesive), "S" refers tosubstrate failure (i.e., fracture or elongation of at least one of thesubstrates), and "M" refers to mixed failure (i.e., a combination ofsubstrate failure and failure within the adhesive bond). The mostpreferred failure modes are substrate failure and mixed failure.

Except as noted below, in each example the methacrylate monomer wasmethylmethacrylate, the acrylate monomer was n-butylacrylate, theorganoborane amine complex was tripropylborane complexed withmonoethanolamine at a nitrogen atom to boron atom ratio of 1:1, theadditional organoborane was tripropylborane, and the thickener waspolymethylmethacrylate.

                                      TABLE 2                                     __________________________________________________________________________         Wt. %                                       Shear                             Meth- Wt. %      Wt. %                      Strength                                                                             Failure               Example                                                                            acrylate                                                                            Acrylate                                                                            Wt. %                                                                              Organo-                                                                            Acid            Wt. % (MPa)  Mode                  No.  Monomer                                                                             Monomer                                                                             Complex                                                                            borane                                                                             Formula     Wt. %                                                                             Thickener                                                                           PTFE                                                                              PE PTFE                                                                              PE                __________________________________________________________________________    14   61.5  26.0  0.5  1.5  SnCl.sub.4  0.5 10.0  0.6 1.5                                                                              A   S                 15   54.5  23.0  0.5  1.5  SnCl.sub.4  0.5 20.0  5.2 10.2                                                                             A   S                 16   49.1  21.4  0.5  1.5  SnCl.sub.4  0.5 27.0  5.2 12.0                                                                             A   S                 17   42.1  18.4  0.5  1.5  SnCl.sub.4  0.5 37.0  5.3 12.0                                                                             A   S                 18   49.1  21.4  0.5  1.5  SnCl.sub.4  0.5 27.0  5.2 11.7                                                                             A   S                 19   49.1  21.4  0.5  1.5  SnCl.sub.4  0.5 27.0.sup.(1)                                                                        5.2 11.7                                                                             A   S                 20   49.1  21.4  0.5  1.5  SnCl.sub.4  0.5 27.0.sup.(2)                                                                        5.3 11.2                                                                             A   S                 21   54.5  23.0  0.5  1.5  SnCl.sub.4  0.5 20.0.sup.(3)                                                                        5.3 11.9                                                                             A   S                 22   54.5  23.0  0.5  1.5  SnCl.sub.4  0.5 20.0.sup.(2)                                                                        4.9 10.2                                                                             A   S                 23   54.5  23.0.sup.(4)                                                                        0.5  1.5  SnCl.sub.4  0.5 20.0  5.2 12.2                                                                             A   S                 24   54.5  23.0.sup.(5)                                                                        0.5  1.5  SnCl.sub.4  0.5 20.0  5.2 10.4                                                                             A   S                 25   54.5.sup.(6)                                                                        23.0  0.5  1.5  SnCl.sub.4  0.5 20.0  5.0 9.9                                                                              A   S                 26   37.2  24.8  5.0  1.0  HCl.sup.(8) 5.0 27.0  5.1 11.2                                                                             A   S                 27   39.9  26.6  3.0  0.5  HCl.sup.(8) 3.0 27.0  1.5 11.4                                                                             A   S                 28   37.7  25.2  5.0  0.1  SnCl.sub.4  5.0 27.0  3.2 12.0                                                                             A   S                 29   40.1  26.8  3.0  1.0  SnCl.sub.4  3.0 27.0  5.0 11.5                                                                             A   S                 30   47.7  18.5  0.5  3.0  SnCl.sub.4  0.5 30.0  5.3 12.4                                                                             A   S                 31   37.7  25.2  5.0  0.1  TiCl.sub.4  5.0 27.0  3.0 11.9                                                                             A   S                 32   37.2  24.8  5.0  1.0  H.sub.2 SO.sub.4                                                                          5.0 27.0  4.2 10.8                                                                             A   S                 33   38.1  25.9  5.0  1.0  H.sub.3 PO.sub.4                                                                          3.0 27.0  4.0 10.5                                                                             A   S                 34   39.8  26.2  3.0  1.0  H.sub.3 PO.sub.4                                                                          3.0 27.0  4.5 11.3                                                                             A   S                 35   13.9  63.6  0.5  1.5  H.sub.3 PO.sub.4                                                                          0.5 20.0  5.3 12.2                                                                             A   S                 36   47.5  20.0  0.5  1.5  H.sub.3 PO.sub.4                                                                          0.5 30.0  5.3 11.5                                                                             A   S                 37   38.0  38.5  1.0  1.5  SnCl.sub.4  1.0 20.0  4.8 12.1                                                                             A   S                 38   69.0  5.0   0.5  5.0  SnCl.sub.4  0.5 20.0  3.6 9.1                                                                              A   S                 39   72.5  5.0   0.5  1.5.sup.(8)                                                                        SnCl.sub.4  0.5 20.0  5.1 11.9                                                                             A   S                 40   44.1  29.4  3.0  0.5  CH.sub.3 COOH                                                                             3.0 20.0  4.2 11.2                                                                             A   S                 41   44.1  29.4  3.0  0.5  CH.sub.2 ═CCOOH                                                                       3.0 20.0  3.6 10.8                                                                             A   S                 42   44.1  29.4  3.0  0.5  CH.sub.2 ═C(CH.sub.3)--COOH                                                           3.0 20.0  3.2 10.5                                                                             A   S                 43   49.6  21.4  0.0  2.0  SnCl.sub.4  2.0 25.0  0.5 1.1                                                                              A   A                 44   51.0  20.4  0.0  1.2  SnCl.sub.4  0.4 27.0  0.8 1.3                                                                              A   A                 45   50.7  20.3  0.0  1.6  SnCl.sub.4  2.4 25.0  0.8 1.2                                                                              A   A                 46   52.1  20.9  0.0  1.6  SnCl.sub.4  0.4 25.0  0.7 1.1                                                                              A   A                 47   47.2  17.8  0.0  7.0  SnCl.sub.4  3.0 25.0  0.1 0.4                                                                              A   A                 48   47.6  16.4  0.0  4.0  SnCl.sub.4  7.0 25.0  0.1 0.2                                                                              A   A                 49   49.6  21.4  2.0  0.0  SnCl.sub.4  2.0 25.0  0.5 1.1                                                                              A   A                 50   68.1  30.0  0.0  1.4.sup.(8)                                                                        NA          0.0 0.0   0.0 0.0                                                                              NA  NA                51   49.0  0.0   0.0  2.0.sup.(8)                                                                        NA          0.0 49.0  0.0 0.0                                                                              NA  NA                52   50.0  21.2  0.4  0.05 SnCl.sub.4  0.4 27.0  0.3 0.9                                                                              A   A                 53   61.0  31.5  0.5  1.5  SnCl.sub.4  0.5 5.0   0.1 0.3                                                                              A   A                 __________________________________________________________________________     NA = Not applicable                                                           .sup.1 Quartz powder                                                          .sup.2 Polystyrenebutylene                                                    .sup.3 Fumed silica                                                           .sup.4 Methylacrylate                                                         .sup.5 1,1,5 Trihydrooctafluoroamylacrylate                                   .sup.6 Butylmethacrylate                                                      .sup.7 1.1 Molar                                                              .sup.8 Tributylborane                                                    

Examples 14 to 17 show the effect of varying the relative amounts of themethacrylate monomer, the acrylate monomer, and the thickening agent. Inexample 14, insufficient thickener was used for this compositionresulting in a runny acrylic adhesive that did not develop very goodoverlap shear strength to polytetrafluoroethylene and polyethylene. Theremaining examples demonstrated much improved adhesion. About 20 to 40wt. % of a polymethylmethacrylate thickener may be usefully incorporatedinto the compositions of the invention. Whether or not a thickener isrequired depends upon the selection of the acrylic monomers. If monomersof a high enough viscosity and a high enough vapor pressure are used,then a thickener may not be needed.

Examples 16 and 18 to 20 show the effect of varying the type ofthickener in otherwise identical formulas. (The polymethylmethacrylateof example 18 was from a different source.) The adhesion to polyethyleneand polytetrafluoroethylene changed only slightly. In addition topolymethylmethacrylate thickeners, quartz powder, fumed silica, andpolystyrene-butylene may also be used. Examples 15, 21, and 22 may besimilarly compared. There was little change in adhesion when thesedifferent thickeners were employed.

Examples 15, 23 and 24 show that changing the acrylate monomer fromn-butylacrylate to methylacrylate or1,1,5-trihydrooctafluoroamylacrylate did not significantly affect theoverlap shear strength to polytetrafluoroethylene or polyethylene.Examples 15 and 25 illustrate that both methylmethacrylate andbutylmethacrylate monomers may be successfully incorporated intopolymerizable compositions according to the invention.

Examples 26 to 39 show the effect on overlap shear strength of bondsmade to polytetrafluoroethylene and polyethylene as a consequence ofchanging the relative amounts of the methacrylate monomer, the acrylatemonomer, the organoborane amine complex, the organoborane, and thethickener. Example 39 uses tributylborane rather than tripropylboranethereby evidencing that alternative alkylboranes may be used. Alsodemonstrated is the effect of using various acids (tin chloride,titanium chloride, hydrochloric acid, sulfuric acid, and phosphoricacid) in differing amounts. Examples 30 and 35, though differing invarious aspects, each afforded excellent adhesion to bothpolytetrafluoroethylene and polyethylene. Polytetrafluoroethylene, ahistorically very difficult material to bond, showed more sensitivity tovariations in the composition than did polyethylene. Varying therelative amounts of the organoborane amine complex, the organoborane,and the acid more than doubled the adhesion to polytetrafluoroethylenewhile having a significantly smaller effect on the adhesion topolyethylene.

Examples 40 to 42 show that in addition to the inorganic acids employedin the previous examples, various organic acids (e.g., acetic acid,acrylic acid and methacrylic acid) can also be used in the polymerizablecompositions of the invention. Organic acids are preferred because theyare easier to handle.

Examples 43 to 48 demonstrate the effect of preparing various adhesivecompositions that do not include an organoborane amine complex. Onlyminimal adhesion to polyethylene and polytetrafluoroethylene wasobtained.

Example 49 illustrates that when an organoborane amine complex based onmonoethanolamine is included but no additional source of organoborane isprovided, acceptable adhesion to polyethylene is obtained but onlyminimal polytetrafluoroethylene adhesion is observed. Thus, the presenceof an additional source of substantially uncomplexed organoborane isneeded for excellent adhesion to polytetrafluoroethylene but notpolyethylene when the complex is based on monoethanolamine. Theorganoborane may be provided by any of the organoboranes described abovein conjunction with the complex. Those organoboranes that are preferredfor use in the complex are also preferred for use as the source ofadditional organoborane. Preferably the amount of substantiallyuncomplexed organoborane is about 0.1 to 7 mole % based on the number ofmoles of acrylic functionality, more preferably about 0.2 to 6 mole %,and most preferably about 1 to 3 mole %. However, as will be shownbelow, by changing the amine to 1,6-hexamethylenediamine, excellentadhesion to polytetrafluoroethylene is possible even when no additionalorganoborane source is provided.

Examples 50 and 51 show the result of providing adhesive compositionsthat include neither an organoborane amine complex nor an acid as wellas the optional thickener (example 50) or the optional acrylate monomer(example 51). No adhesion to either polyethylene orpolytetrafluoroethylene was obtained. The compositions of these examplesincluded tributylborane.

The compositions of examples 52 and 53 show the effect of using toolittle organoborane (example 52) and too little thickener (example 53).

From the foregoing examples it can be seen that a particularly desirablepolymerizable composition according to the invention comprises, based onthe total weight of the composition, about 5 to 65 wt. % of an alkylacrylate monomer (preferably butyl acrylate), about 0.5 to 5 wt. % of anorganoborane amine complex (preferably atripropylborane-monoethanolamine complex), about 0.1 to 5 wt. % of anadditional organoborane (preferably tripropyl borane), about 0.5 to 5wt. % of an acid, about 20 to 40 wt. % of a thickening agent (preferablypolymethylmethacrylate), the balance (about 10 to 65 wt. %) being analkyl methacrylate (preferably methylmethacrylate).

EXAMPLE 54

An adhesive composition that is especially well suited for bondingfluorinated polymers and polyethylene was prepared by blending 42.1 wt.% methylmethacrylate, 18.4 wt. % butylacrylate, and 37.0 wt. % of apolymethylmethacrylate thickening agent until complete dissolutionoccurred. 0.5 wt. % of SnCl₄ acid was then added followed by a mixtureof 1.5 wt. % tripropylborane and 0.5 wt. % monoethanolamine. Once mixed,the composition was applied to a polyethylene substrate and apolytetrafluoroethylene substrate. The two substrates were joined, curedfor 24 hours under ambient conditions, and tested for overlap shearstrength according to GOST 14759-69, in the manner described above. Theoverlap shear strength was 5.1 MPa with substrate failure.

EXAMPLES 55 TO 75

Utilizing the weight percentages shown in Table 3 below, a series ofadhesive compositions according to the invention was prepared byblending methylmethacrylate monomer, n-butyl acrylate monomer, and apolymethylmethacrylate thickening agent until complete dissolutionoccurred. An acid followed by an organoborane amine complex were thenadded with stirring. Except as noted below, the acid was methacrylicacid and the organoborane amine complex was based on tripropylborane and1,6-hexamethylenediamine (1:1 nitrogen atom to boron atom ratio).Following the procedures of GOST 14759-69 as described above, bondedcomposites using polyethylene (PE), polytetrafluoroethylene (PTFE) andpolyvinyl chloride (PVC) substrates (each substrate being bonded toanother substrate of the same material) were prepared, cured underambient conditions for 48 hours, and tested for overlap shear strengthwith the results reported in Table 3 below. Also shown in Table 3 is thefailure mode of the bonded composites, as defined above.

                                      TABLE 3                                     __________________________________________________________________________         Wt. %                                                                         Methyl-                                                                              Wt. %  Wt. %             Shear Strength                           Example                                                                            methacrylate                                                                         Butylacrylate                                                                        Organoborane                                                                          Wt. %                                                                             Wt. % (MPa)     Failure Mode                   No.  Monomer                                                                              Monomer                                                                              Amine Complex                                                                         Acid                                                                              Thickener                                                                           PE PTFE                                                                              PVC                                                                              PTFE                                                                              PE PVC                     __________________________________________________________________________    55   40.8   28.2   0.5     0.5 30.0  12.1                                                                             8.8 11.5                                                                             S   S  S                       56   37.9   27.2   2.9     2.9 29.1  11.8                                                                             9.0 11.7                                                                             S   S  S                       57   36.2   25.7   4.8     4.8 28.6  11.5                                                                             9.3 11.3                                                                             S   S  S                       58   34.6   24.3   6.5     6.5 28.0  10.9                                                                             8.4 10.7                                                                             S   S  S                       59   35.6   28.7   3.0     3.0 29.7  12.2                                                                             10.1                                                                              11.7                                                                             S   S  S                       60   39.8   30.1   2.9     2.9 24.3  10.8                                                                             9.5 10.5                                                                             S   S  S                       61   30.0   27.0   3.0     3.0 37.0  11.7                                                                             9.7 11.4                                                                             S   S  S                       62   39.3   13.1   6.5     6.5 34.6  11.5                                                                             8.1 10.3                                                                             M   S  S                       63   35.6   38.6   0.5     0.5 24.8  10.5                                                                             7.8 9.6                                                                              M   S  S                       64   11.8   52.9   2.9     2.9 29.4  11.0                                                                             8.9 10.4                                                                             S   S  S                       65   36.9   28.2   2.9.sup.(1)                                                                           2.9 29.1  11.2                                                                             9.2 10.8                                                                             S   S  S                       66   36.9   28.2   2.9     2.9.sup.(2)                                                                       29.1  10.7                                                                             8.4 10.8                                                                             S   S  S                       67   36.9   28.2   2.9     2.9 29.1  12.3                                                                             9.9 11.0                                                                             S   S  S                       68   36.9   28.2   2.9     2.9 29.1  10.8                                                                             9.3 11.6                                                                             S   S  S                       69   36.9   28.2   2.9.sup.(3)                                                                           2.9 29.1  11.2                                                                             9.3 10.1                                                                             S   S  S                       70   36.9   28.2   2.9.sup.(4)                                                                           2.9 29.1  11.4                                                                             9.0 10.5                                                                             S   S  S                       71   41.5   27.9   0.4     0.4 29.9  5.3                                                                              3.2 4.3                                                                              A   A  S                       72   31.2   24.8   8.3     8.3 27.5  1.2                                                                              0.7 1.5                                                                              A   A  A                       73   44.7   30.1   2.9     2.9 19.4  3.6                                                                              2.3 3.8                                                                              A   A  A                       74   9.9    54.5   3.0     3.0 29.7  3.5                                                                              2.0 3.1                                                                              A   A  A                       75   52.1   10.4   3.1     3.1 31.3  2.5                                                                              1.8 2.9                                                                              A   A  A                       __________________________________________________________________________     .sup.1 Complex based on triisobutylborane and 1,6hexamethylenediamine         (nitrogen atom:boron atom ratio = 1:1)                                        .sup.2 Acrylic acid                                                           .sup.3 Complex based on trin-butylborane and 1,6hexamethylenediamine          (nitrogen atom:boron atom ratio = 1:1)                                        .sup.4 Nitrogen atom:boron atom ratio = 2:1.                             

Examples 55 to 59 show the effect on adhesion to polyethylene,polytetrafluoroethylene and polyvinylchloride as the relative amounts ofmethylmethacrylate monomer, butylacrylate monomer, methacrylic acid, andpolymethylmethacrylate thickening agents are varied. Examples 60 to 64make similar comparisons and also vary the amount of complex.

Examples 65 to 70 demonstrate the results of varying the organoboraneamine complex, the acid and the thickening agent. In example 65 theorganoborane amine complex is based on tri-isobutylborane and1,6-hexamethylenediamine at a 1:1 nitrogen atom to boron atom ratio. Inexample 66, the acid is acrylic acid. Examples 67 and 68 employpolymethylmethacrylate thickening agents from different sources.

Example 69 uses an alkylborane amine complex based on tri-n-butylboraneand 1,6-hexamethylenediamine (nitrogen atom to boron atom ratio=1:1) Theorganoborane amine complex of example 70 is based on tripropylborane and1,6-hexamethylenediamine but at a nitrogen atom to boron atom ratio of2:1.

Examples 71 and 72 show the effect of having too little or too muchcomplex and too little or too much acid in the adhesive composition.Example 73 illustrates that poor adhesion is obtained when insufficientthickening agent is employed. Consequently the adhesive composition ofexample 73 was too low in viscosity and began to oxidize prematurely.Example 73 may be contrasted with example 15 where the presence ofadditional organoborane overcomes the premature oxidation. Example 74demonstrates the use of a relatively small amount of methacrylatemonomer with a relatively large amount of acrylate monomer. Example 75shows the opposite relationship.

From the foregoing examples it can be seen that a particularly desirablepolymerizable composition according to the invention comprises, based onthe total weight of the composition, about 10 to 55 wt. % of an alkylacrylate (preferably butyl acrylate), about 10 to 50 wt. % of an alkylmethacrylate (preferably methyl methacrylate), about 0.5 to 7 wt. % ofan organoborane amine complex (preferably atripropylborane-1,6-hexamethylenediamine complex), about 0.5 to 5 wt. %of an acid (preferably acrylic or methacrylic acid), and about 25 to 40wt. % of a thickening agent (preferably polymethylmethacrylate).

EXAMPLES 76 TO 98

Examples 76 to 98 illustrate another preferred way in which thepolymerization initiator systems of the invention may be used. In theseexamples, the substrates to be bonded were pretreated (e.g., by sprayingor brushing) with a primer that comprised an organoborane amine complexin an organic solvent. Once applied, the primer solvent was evaporatedand a polymerizable acrylic adhesive composition was then applied. Thesubstrates were then mated and allowed to cure for 24 to 48 hours beforethey were tested for overlap shear strength following the procedures ofGOST 14759-69, as described above. The results of these tests usingpolyethylene (PE), polyvinylchloride (PVC) and polytetrafluoroethylene(PTFE) substrates, each substrate bonded to a second substrate of thesame material, are reported below in Table 4. The "exposure time" refersto the time for which the primer was exposed to air after having beenapplied to a substrate and before the adhesive was applied.

More specifically, and unless otherwise noted below: the organoboraneamine complex was based on tripropylborane and 1,6-hexamethylenediamineat a nitrogen atom to boron atom ratio (N:B) of 1:1.; the complex wasdissolved in pentane (solvent) to a 10% solution; and the polymerizableadhesive comprised 39 wt. % methylmethacrylate monomer, 35 wt. %butylacrylate monomer, 1 wt. % methacrylic acid, and 25 wt. % of apolymethylmethacrylate thickening agent.

                  TABLE 4                                                         ______________________________________                                               Primer      Exposure  Shear Strength                                   Example                                                                              Composition Time      (MPa)                                            No.    Remarks     (Min.)    PE   PVC    PTFE                                 ______________________________________                                        76     5% Solution 10        9.4  8.2    8.2                                  77     8% Solution 10        10.1 8.8    9.5                                  78     No Remarks  10        10.2 8.2    9.4                                  79     12% Solution                                                                              10        10.3 8.7    9.6                                  80     No Remarks  30        12.1 9.1    9.6                                  81     No Remarks  60        11.3 8.8    9.5                                  82     No Remarks  120       10.8 8.9    9.4                                  83     No Remarks  180       10.5 8.7    9.8                                  84     No Remarks  360       9.8  8.5    8.5                                  85     No Remarks  420       9.7  8.6    8.3                                  86     No Remarks  480       5.7  5.3    4.3                                  87     N:B = 2.5:1 20        9.8  8.5    8.4                                  88     N:B = 2:1   20        10.1 8.2    8.7                                  89     N:B = 1.3:1 20        10.5 8.8    9.1                                  90     N:B = 0.9:1 20        9.7  8.4    8.9                                  91     Complex uses                                                                              20        11.3 8.9    9.5                                         triisobutyl-                                                                  borane and 1,6-                                                               hexamethylene                                                                 diamine                                                                92     Complex uses                                                                              20        10.3 8.0    8.5                                         triisobutyl-                                                                  borane and 1,6-                                                               hexamethylene                                                                 diamine,                                                                      N:B = 2:1                                                              93     Hexane solvent                                                                            20        11.5 8.9    9.1                                  94     Petroleum   20        11.3 8.7    9.0                                         ether solvent                                                          95     White spirits                                                                             20        11.1 8.6    9.2                                         solvent                                                                96     Benzene     20        10.9 8.4    9.5                                         solvent                                                                97     Toluene solvent                                                                           20        11.0 8.5    8.7                                  98     Butylacetate                                                                              20        10.7 8.3    8.4                                         solvent                                                                ______________________________________                                    

Examples 76 to 79 illustrate that primer solution concentrations ofabout 5% to 12% may be used in accordance with the invention. Examples78 and 80 to indicate that once the primer has been applied, the primedsubstrate can be left exposed to the air for up to at least 7 hourswithout detrimentally affecting the strength of the subsequent adhesivebond. In examples and 87 to 90 the nitrogen atom to boron atom ratio inthe organoborane amine complex was varied from 0.9:1 to 2.5:1 withoutadversely affecting adhesion.

Examples 91 and 92 indicate that useful organoborane amine complexes canbe prepared from tri-isobutylborane and 1,6-hexamethylenediamine.Examples to 98 illustrate the wide variety of useful organic solventsthat may be employed in preparing primer compositions according to theinvention.

EXAMPLES 99 TO 106

Examples 99 to 106 further illustrate the provision and use of primersaccording to the invention. Primers were prepared and applied asdescribed in conjunction with examples 76 to 98 (tripropylborane and1,6-hexamethylenediamine in pentane). The ratio of nitrogen atoms toboron atoms (N:B) was 1:1 except in example 99 (N:B=4:1) and example 100(N:B=0.8:1). The concentration of the primer solution, the exposuretime, the "cure time" (i.e., the time over which the bonded compositeswere cured before testing), and the overlap shear strength test results(based on GOST 14759-69) are all reported in Table 5 below.

The bonded composites were prepared as described in conjunction withExamples 76 to 98 and using the adhesive composition of these examples.

                  TABLE 5                                                         ______________________________________                                               Concentra-                                                                    tion of                                                                       Primer    Exposure        Shear Strength                               Example                                                                              Solution  Time     Cure   (MPa)                                        No.    (wt. %)   (Min.)   Time   PE   PVC  PTFE                               ______________________________________                                         99    10        20       24 hrs.                                                                              2.1  2.3  1.7                                100    10        20       24 hrs.                                                                              2.5  2.7  1.9                                101    3         20       24 hrs.                                                                              3.2  2.5  1.3                                102    15        20       24 hrs.                                                                              5.6  4.9  4.1                                103    15        420      60 hrs.                                                                              9.6  8.5  8.4                                104    15        420      10 days                                                                              9.7  8.4  8.3                                105    15        420      30 days                                                                              9.8  8.2  8.5                                106    15        420      6 months                                                                             9.5  8.3  8.6                                ______________________________________                                    

Example 99 shows the effect on adhesion when the nitrogen atom to boronatom ratio is too high (4:1) and example 100 shows the effect when theratio is too low (0.8:1). The ratio should be in the range of about 1:1to 2:1, more preferably about 1:1 to less than 2:1, even more preferablyabout 1:1 to 1.5:1, and most preferably about 1:1.

Example 101 shows that reduced adhesion results from using a primerhaving a reduced concentration of the organoborane amine complex.Adhesion improved significantly when the primer solution concentrationwas increased from 3% (example 101) to 15% (example 102).

Examples 103 through 106 demonstrate that the primer solutions of theinvention can be exposed to air (after application to a substrate) forup to at least 7 hours without adversely affecting subsequently madeadhesive bonds. The adhesive bonds show no significant diminution inshear strength even after aging for 6 months before testing.

EXAMPLES 107 TO 110

Examples 107 to 110 illustrate the effect of using a primer with apolymerizable acrylic composition that also contains an organoboraneamine complex.

In examples 107 and 108 a 10% primer solution comprising atripropylborane-1,6-hexamethylenediamine complex in pentane was appliedto polyethylene (PE), polyvinylchloride (PVC) andpolytetrafluoroethylene (PTFE) substrates as described in conjunctionwith examples 99 to 106. A polymerizable acrylic adhesive comprising44.1 wt. % methylmethacrylate monomer, 29.4 wt. % butylacrylate monomer,3.0 wt. % methacrylic acid, 20 wt. % Polymethylmethacrylate thickeningagent, 0.5 wt. % tripropylborane, and 3.0 wt. %tripropylborane-monoethanolamine complex (N atom:B atom ratio=1:1) wasprepared and allowed to remain in the mixing vessel for a period of timereferred to in Table 6 below as the "Pot Time." The acrylic adhesivecomposition was then applied to the primed substrates. In each examplethe acrylic adhesive was applied 10 minutes after the primer had beenapplied. Bonded composites were prepared, cured, and tested according toGOST 14759-69 and Table 6 below. The overlap shear strength test resultsare also reported in Table 6.

Examples 109 and 110 were prepared and tested in the same manner exceptthat no primer was applied to the substrates.

                  TABLE 6                                                         ______________________________________                                                                   Shear Strength                                     Example                                                                              Primer     Pot Time (MPa)                                              No.    Applied    (Min.)   PE     PVC  PTFE                                   ______________________________________                                        107    Yes        20       10.3   8.7  8.4                                    108    Yes        30       10.5   9.1  8.7                                    109    No         20       1.0    0.8  0.0                                    110    No         5        10.5   3.2  0.0                                    ______________________________________                                    

Examples 107 to 110 show that the use of a primer according to theinvention can extend the working life of compositions that also includea polymerization initiator. The useful working life of the adhesive ofexamples 109 and 110 was between 5 and 20 minutes for polyethylene andpolyvinylchloride and less than 5 minutes for polytetrafluoroethylene.However, with the use of a primer, the working life could be extended tomore than 30 minutes.

EXAMPLES 111 TO 114

Examples 111 to 114 are similar to examples 107 to except using adifferent primer and a different adhesive. The primer of these examples(applied only in examples 111 and 112) is similar to that of examplesand 108 except that the nitrogen atom to boron atom ratio is 1.3:1. Theexposure time was 60 minutes. The polymerizable acrylic adhesivecomprised 40.8 wt. % methylmethacrylate monomer, 27.2 wt. %butylacrylate monomer, 1.0 wt. % methacrylic acid, 30.0 wt. %polymethylmethacrylate thickening agent, and 1.0 wt. % of atripropylborane-1,6-hexamethylenediamine complex (nitrogen atom:boronatom ratio=1:1). Bonded composites were prepared and tested as describedabove in examples 107 to 110 with the results reported below in Table 7.

                  TABLE 7                                                         ______________________________________                                                                   Shear Strength                                     Example                                                                              Primer     Pot Time (MPa)                                              No.    Applied    (Min.)   PE     PVC  PTFE                                   ______________________________________                                        111    Yes        60       10.7   8.4  8.4                                    112    Yes        120      10.3   8.1  8.7                                    113    No         20       0.9    1.2  0.7                                    114    No         5        12.1   8.8  11.5                                   ______________________________________                                    

With the use of a primer, the working life of the adhesive compositionsof these examples could be extended from less than 20 minutes to morethan 2 hours.

EXAMPLES 115 AND 116

A series of polymerizable acrylic monomer compositions was prepared toevaluate the utility of different amines in providing the organoboraneamine complex. Each composition comprised 19.2 wt. % n-butylacrylate,55.3 wt. % methylmethacrylate, 22.4 wt. % polymethylmethacrylatethickening agent, 0.8 wt. % methacrylic acid, and 2.2 wt. % of atripropylborane amine complex having a nitrogen atom to boron atom ratioof 1:1. The various amines used along with the overlap shear strengthtest results on polyethylene (PE) and polytetrafluoroethylene (PTFE) (24hour cure) are shown below in Table 8.

                  TABLE 8                                                         ______________________________________                                                          Shear Strength (MPa)                                        Example No.                                                                              Amine        PE       PTFE                                         ______________________________________                                        115        Aniline      1.5      0                                            116        Triethylamine                                                                              10.9     0                                            ______________________________________                                    

These examples did not demonstrate any adhesion topolytetrafluoroethylene. Moreover, the organoborane amine complexes werepyrophoric when tested according to the charring time and ignition timetests described above and hence were considered unsuitable.

EXAMPLES 117 AND 118

Two polymerizable compositions containing a methacrylate monomer but noacrylate monomer were prepared as shown below in Table 9. In eachexample the methacrylate monomer was methylmethacrylate, the thickeningagent was polymethylmethacrylate, the acid was methacrylic acid, and theorganoborane amine complex was based on hexamethylenediamine andtripropylborane (nitrogen atom to boron atom ratio=1:1). Bondedcomposites using polyethylene and polytetrafluoroethylene were preparedas described above and cured for 24 hours under ambient conditionsbefore testing for overlap shear strength, as shown in Table 9.

                  TABLE 9                                                         ______________________________________                                                                Wt. %                                                        Wt. %            Organo-      Shear                                           Methyl           borane Wt. % Adhesion                                 Example                                                                              Meth-    Wt. %   amine  Thick-                                                                              (MPa)                                    No.    acrylate Acid    complex                                                                              ener  PE    PTFE                               ______________________________________                                        117    52.2     6.6     6.6    34.6  10.7  2.3                                118    56.3     3.2     3.2    37.4  0     0                                  ______________________________________                                    

Examples 117 and 118 show that in polymerizable compositions comprisingonly methacrylate monomer it may be necessary to use additionalorganoborane amine complex and acid to achieve acceptable adhesion topolyethylene and polytetrafluoroethylene.

EXAMPLES 119 AND 120

Examples 119 and 120 describe additional ways in which primers accordingto the invention may be provided. Example 119 comprised 0.5 wt. %bis(tributylperoxy)triphenylantimony, 73.1 wt. % methylmethacrylatemonomer and 25.4 wt. % butylacrylate monomer. The resulting compositionwas degassed and then 1.0 wt. % of tripropylboron was added thereto. Apolytetrafluoroethylene coupon was then primed with this composition andbonded to a like coupon with a polyurethane adhesive. The overlap shearstrength (when tested as described above) was 6.3 MPa. An unprimedcontrol example showed no adhesion. The peroxide provides a source ofoxygen since the composition was degassed. If the composition had notbeen degassed, atmospheric oxygen would have sufficed as the oxygensource and the addition of peroxide would have been unnecessary.

In example 120, a polyethylene coupon was primed with a compositioncomprising 39 wt. % methylmethacrylate, 25 wt. % n-butylacrylate, 30 wt.% polymethylmethacrylate, 3 wt. % of an organoborane amine complex(1,6-hexamethylenediamine and tripropylborane at a 1:1 nitrogen atom toboron atom ratio), and 3 wt. % methacrylic acid. When bonded to a likesubstrate with an epoxy adhesive, the bonded composite exhibited anoverlap shear strength (when tested as described above) of 6.0 MPa. Anunprimed control example showed no adhesion.

Thus compositions comprising an acrylic monomer, an organoborane, and anoxygen source or comprising an acrylic monomer, an organoborane aminecomplex, and an acid can be used to prime fluoroplastics for improvedadhesion to subsequently applied adhesives.

EXAMPLES 121 TO 125

Examples 121 to 125 show the effect of the nitrogen atom to boron atomratio (N:B) on the performance of polymerizable acrylic compositionsaccording to the invention. A series of organoborane amine complexesbased on 1,6-hexamethylenediamine and tri-n-butylborane at variousnitrogen atom to boron atom ratios (as shown below in Table 10) wasprepared. 0.186 g of the complex was added to 5 g a polymerizableacrylic composition made from 78 g methylmethacrylate monomer, 56 g2-butylacrylate monomer, 60 g of a medium molecular weightpolymethylmethacrylate thickening agent, and 6 g of methacrylic acid.

Bonded composites based on polyethylene (PE), polypropylene (PP), andpolytetrafluoroethylene (PTFE) (each substrate bonded to a substrate ofthe same material) having a 161 mm² overlap area and 0.15 mm bondlinethickness were prepared, fixtured with adhesive tape and binder clips,and cured under ambient conditions for 24 hours. The substrates measuredabout 25 mm×100 mm×3 mm thick. The bonded composites were then tested tofailure in a tensile testing machine using a crosshead speed of 2.5 mmper minute. Results are shown below in which the reported values are anaverage of 3 samples. Examples 121 and 122 demonstrated substratefailure. The remaining examples failed adhesively.

                  TABLE 10                                                        ______________________________________                                                      Overlap Shear                                                                 Strength (MPa)                                                  Example No. N:B     PE         PP   PTFE                                      ______________________________________                                        121         1:1     4.9        4.1  1.9                                       122         1.5:1   5.3        3.0  1.4                                       123         2:1     2.4        2.5  0.3                                       124         3:1     0.3        0.1  0.0                                       125         4:1     0.3        0.2  0.0                                       ______________________________________                                    

These examples show the surprising and unexpected improvement inadhesion to various low energy substrates that is possible when usingthe polymerizable compositions of the invention.

Numerous variations and modifications are possible within the scope ofthe foregoing specification without departing from the spirit of theinvention which is defined in the accompanying claims.

What is claimed is:
 1. A polymerizable acrylic compositioncomprising:(a) at least one acrylic monomer; (b) an effective amount ofan organoborane amine complex having the structure: ##STR15## wherein:R¹ is an alkyl group having 1 to 10 carbon atoms; R² and R³ areindependently selected from the group consisting of alkyl groups having1 to 10 carbon atoms and phenyl-containing groups; R⁴ is selected fromthe group consisting of CH₂ CH₂ OH and (CH₂)_(x) NH₂ wherein x is aninteger greater than 2; R⁵ is hydrogen or an alkyl group having 1 to 10carbon atoms; and the nitrogen atom to boron atom ratio is about 1:1 to1.5:1; and (c) an effective amount of an acid for initiatingpolymerization of the acrylic monomer.
 2. A polymerizable acryliccomposition according to claim 1 wherein R¹, R² and R³ are eachindependently selected from the group consisting of alkyl groups having2 to 5 carbon atoms.
 3. A polymerizable acrylic composition according toclaim 2 wherein R⁴ is selected from the group consisting of CH₂ CH₂ OHand (CH₂)_(x) NH₂ wherein x is an integer from 2 to 6 and furtherwherein R⁵ is hydrogen.
 4. A polymerizable acrylic composition accordingto claim 3 wherein R⁴ is selected from the group consisting of CH₂ CH₂OH and (CH₂)₆ NH₂.
 5. A polymerizable acrylic composition according toclaim 1 wherein the nitrogen atom to boron atom ratio is about 1:1.
 6. Apolymerizable acrylic composition according to claim 1 wherein the atleast one acrylic monomer is a blend of an alkyl acrylate monomer and analkyl methacrylate monomer.
 7. A polymerizable acrylic compositionaccording to claim 6 wherein the alkyl acrylate monomer is a butylacrylate and the alkyl methacrylate monomer is methyl methacrylate.
 8. Apolymerizable acrylic composition according to claim 1 wherein theorganoborane amine complex comprises about 0.15 to 5 mole %, based onthe number of moles of acrylic functionality.
 9. A polymerizable acryliccomposition according to claim 1 further comprising a thickening agentfor increasing the viscosity of the composition.
 10. A polymerizableacrylic composition according to claim 1 wherein R⁴ is CH₂ CH₂ OH, theacrylic composition further comprising about 0.1to 7 mole %, based onthe number of moles of acrylic functionality, of a substantiallyuncomplexed trialkylborane.
 11. A polymerizable acrylic compositionaccording to claim 1 comprising:(a) about 0.5 to 7 wt. % of theorganoborane complex, wherein R¹, R² and R³ are independently selectedfrom the group consisting of alkyl groups having 2 to 5 carbon atoms andfurther wherein R⁴ is (CH₂)₆ NH₂ ; (b) about 0.5 to 7 wt. % of the acid,wherein the acid is either acrylic acid or methacrylic acid; (c) a blendof (i) a butyl acrylate that provides about 10 to 55 wt. % and (ii)methyl methacrylate that provides about 10 to 50 wt. %; and (d) about 25to 40 wt. % of a thickening agent;wherein the sum of a+b+c+d equals 100wt. %.
 12. A polymerizable acrylic composition according to claim 1comprising:(a) about 0.5 to 5 wt. % of the organoborane amine complex,wherein R¹, R² and R³ are independently selected from the groupconsisting of alkyl groups having 2 to 5 carbon atoms and furtherwherein R⁴ is CH₂ CH₂ OH; (b) about 0.5 to 5 wt. % of the acid; (c) ablend of (i) an alkyl acrylate that provides about 5 to 65 wt. % and(ii) about 10 to 65 wt. % of an alkyl methacrylate; (d) about 0.1 to 5wt. % of a substantially uncomplexed trialkylborane, the alkyl groups ofwhich may be the same or different, each having 1 to 10 carbon atoms;and (e) about 20 to 40 wt. % of a thickening agent;wherein the sum ofa+b+c+d+e equals 100 wt. %.